In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned to produce hot combustion gas. An annular stationary shroud forms a tunnel-like gas flow path through which the hot combustion gas passes. A gas turbine is located within the volume defined by the stationary shroud and is mounted on the same shaft as the compressor. The flow of combustion gas turns the gas turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more complex version of the gas turbine engine, the compressor and a high pressure turbine are mounted on one shaft, and the fan and low pressure turbine are mounted on a separate shaft. The hot combustion gas, now an exhaust gas, flows from the back of the engine, driving it and the aircraft forward.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion-gas temperature. The maximum temperature of the combustion gas is normally limited by the materials used to fabricate the stationary shroud and the turbine vanes and turbine blades of the turbine. In current engines, the stationary shroud and the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1900-2150° F.
Many approaches have been used to increase the operating temperature limits of stationary shrouds, turbine blades, turbine vanes, and other hot-section components to their current levels. For example, the composition and processing of the base materials themselves have been improved, and a variety of solidification techniques have been developed to take advantage of oriented grain structures and single-crystal structures. Physical cooling techniques, in which cooling air is directed through small holes in the component, may also be used.
In yet another approach, coatings are applied to the surface of the substrate to inhibit the oxidation of the substrate and to insulate the substrate, thereby permitting the substrate material to be used at a higher temperature than would otherwise be possible. The most widely used coatings are aluminum-rich layers whose surfaces oxidize to an aluminum oxide scale to inhibit further oxidation. The aluminum-rich layer may serve as either an environmental coating or as a bond coat under a thermal-insulator ceramic thermal barrier coating. Other types of coatings have also been used, although with less-satisfactory results.
Protective layers continue to be used to protect substrates, but there is always a need for further improvements to increase the operating temperatures of the coated substrates and to prolong their service lives. The present invention fulfills this need, and further provides related advantages.